Fuel cells are among the most efficient of power generation devices. One type of solid oxide fuel cell (SOFC) has a projected 70 percent net efficiency when used in an integrated SOFC-combustion turbine power system in which the turbine combustor is replaced by a SOFC. Conventional SOFCs operate with various hydrocarbon fuel sources. For example, the SOFCs may operate directly on natural gas at about 1000.degree. C., without the need for external reforming of the natural gas, i.e., the high operating temperature results in internal reforming of the natural gas to H.sub.2 and CO.
An individual fuel cell element of a conventional SOFC design consists of a closed end porous strontium-doped lanthanum manganite tube, which serves as the support structure for the individual cell and is also the cathode or air electrode of the cell. A thin, gas-tight yttria-stabilized zirconia electrolyte covers the air electrode except for a relatively thin strip of an interconnection surface, which is a dense gas-tight layer of magnesium-doped lanthanum chromite. This strip serves as the electric contacting area to an adjacent cell or, alternatively, to a power contact. A porous nickel-zirconia cermet layer, which is the anode, or fuel electrode, covers the electrolyte, but not the interconnection strip. Exemplary fuel cells are disclosed in U.S. Pat. No. 4,431,715 to Isenberg, U.S. Pat. No. 4,490,444 to Isenberg, U.S. Pat. No. 4,562,124 to Ruka, U.S. Pat. No. 4,631,238 to Ruka, U.S. Pat. No. 4,748,091 to Isenberg, U.S. Pat. No. 4,791,035 to Reichner, U.S. Pat. No. 4,833,045 to Pollack et al., U.S. Pat. No. 4,874,678 to Reichner, U.S. Pat. No. 4,876,163 to Reichner, U.S. Pat. No. 5,108,850 to Carlson et al., U.S. Pat. No. 5,258,240 to Di Croce et al., and U.S. Pat. No. 5,273,828 to Draper et al., each of which is incorporated herein by reference.
Conventional SOFC electric generators consist of a containment vessel in which are housed an array of series-parallel electrically connected cells to form submodules, which in turn are further combined in series or parallel connections to form the generator module. Depending on the electrical capacity of the generator module, the number of cells may vary from a few hundred to several thousand individual cells. These modules are equipped with ancillary equipment such as air delivery tubes to each cell and are housed in a metal container equipped with fuel and air delivery systems as well as an exhaust port for unused fuel and excess air.
The SOFC generator container is simultaneously exposed to oxidizing and reducing gas atmospheres. In operation, the wall temperature is about 650.degree. C., although excursions of perhaps 100-150.degree. C. could occur during the operation. This dual gas atmosphere presents a greater corrosion challenge to the container than does exposure to either flowing air, the environment outside the container, or fuel (CH.sub.4, CO, H.sub.2, CO.sub.2, H.sub.2 O), the environment inside the container. The most widely used SOFC container material is stainless steel AISI 304. Corrosion that occurs to such fuel cell containers can significantly limit the performance and lifetime of a fuel cell system.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.